There are many applications in industry that require seals and sealing assemblies which can withstand harsh environments such as extreme heat or cold, chemical attack, and exposure to high radioactivity levels. In most of these environments, elastomeric seals, plastic seals and even composite seals cannot be employed.
Currently there are available materials such as graphite or carbon fiber, aluminum silicate based refractory fiber, etc. which can be used in certain of these harsh environments. It is to be understood that while certain materials may be used in one harsh environment, e.g., extremely high temperatures it may not be suitable in another harsh environment, e.g., exposure to high radiation levels.
Graphite or carbon fiber seals are commonly used as both static and dynamic seals in equipment, e.g., pumps, valves, actuators used in nuclear facilities, e.g., braided stem packing for valves in a nuclear power plant where it is practical to apply high forces to cause desired sealing or static seals between two fixed surfaces. Dynamic seals typically employ chevron shaped compressed foil type or braided packing rings. Both types require considerable compressive forces to effect a dynamic seal, with the coarse surface of the braided rings requiring considerably more than the compressed foil types due to the unevenness of their surfaces. Both impart a great deal of friction with the stem and body but not so high as to negate their severe service capabilities. However should one desire to use these materials to seal between the piston an cylinder bore of an actuator, the high friction caused by the necessary compressive forces renders the concept unusable as the actuator loses an excessive amount of output force or torque.
While it has been postulated that a carbon or graphite braid made of very fine fibers would provide dynamic sealing without excessive compressive force, to date, such fine braided packings are not available.